The present invention relates in general to amplifier circuits and, more particularly, to a differential amplifier input stage operating with a charge pump supplying a differential transistor pair.
Differential amplifier circuits are used in a myriad of applications to amplify differential signals. The amplifier circuit generally requires one or more power supply potentials for operation. In many amplifiers, the common mode operating range of the differential input signal is limited to a value less than the rails of the power supply potentials. The modern trend is to lower the operating potential to the amplifier, especially for battery-supplied applications such as cellular telephones, digital pagers and portable computers. The lower operating potential is desirable as it requires fewer serially coupled battery cells to supply the operating potential to the end use circuit.
As the operating potential to the amplifier decreases, the signal headroom for common mode operating range of the differential input stage also reduces. A conventional differential amplifier circuit may comprise a differential transistor pair driven by a current source from the positive power supply potential and resistors or current sources between the differential transistor pair and the negative power supply conductor. The headroom needed for the current source and the base-emitter junction potentials of the differential transistor pair is typically 800 millivolts. For an application where the target positive power supply potential is say one volt and the negative power supply potential is ground, there is only 200 millivolts of common mode operating range for the differential input signal. The limited common mode operating range is insufficient for many applications that need low operating potentials.
Hence, a need exists to increase the common mode operating range of the differential input signal while maintaining a low power supply potential to the amplifier.